Broadband source model of the 2021 M_W 7.1 Fukushima‐ken Oki earthquake in Japan based on seafloor and onshore strong‐motion records

Abstract

The 2021 MW 7.1 Fukushima‐ken Oki earthquake was the largest down‐dip compressional intraslab earthquake since August 2016, which is when S‐net records from the seafloor started to be provided. In this study, the empirical Green's function method was used with strong‐motion records from S‐net on the seafloor and KiK‐net onshore to estimate the broadband (0.2–10 Hz) source model of this earthquake. Three strong‐motion generation areas (SMGAs) with very high stress drops were estimated around the edge of the fault. One SMGA with a stress drop of 125 MPa was located in the oceanic crust in the shallow part of the Pacific Plate. The other two SMGAs with stress drops of 313 and 188 MPa were located in the deep part where previous source models without S‐net estimated a small slip. The two deep SMGAs increased the contribution of S‐net to more than that of KiK‐net. The short‐period spectral level was as high as that of the 2011 MW 7.1 Miyagi‐ken Oki earthquake, which was also a down‐dip compressional intraslab earthquake. The estimated broadband source model simulated the horizontal and vertical motions well at stations within 200 km of the hypocenter, except for surface waves at a few S‐net stations.